You choose a value for X which is then reduced by two colourless mana for each conduit you have on the battlefield.
If I have a Conduit of Ruin on the battlefield and say that I will be
casting Endless One for 10 mana, will the cost reduction of 2 mana
from the Conduit's effect reduce the X so that the Endless one is 8/8
or will it remain 10/10?
You choose a value for X and then a slew of effects may either increase or decrease the cost. Here is the pertinent rules. It is a bit long but worth the read.
601.2a To propose the casting of a spell, a player first moves that card (or that copy of a card) from where it is to the stack. It
becomes the topmost object on the stack. It has all the
characteristics of the card (or the copy of a card) associated with
it, and that player becomes its controller. The spell remains on the
stack until it’s countered, it resolves, or an effect moves it
elsewhere.
601.2b If the spell is modal, the player announces the mode choice (see rule 700.2). If the player wishes to splice any cards onto the
spell (see rule 702.46), he or she reveals those cards in his or her
hand. If the spell has alternative or additional costs that will be
paid as it’s being cast such as buyback or kicker costs (see rules
117.8 and 117.9), the player announces his or her intentions to pay any or all of those costs (see rule 601.2f). A player can’t apply two
alternative methods of casting or two alternative costs to a single
spell. If the spell has a variable cost that will be paid as it’s
being cast (such as an {X} in its mana cost; see rule 107.3), the
player announces the value of that variable. If a cost that will be
paid as the spell is being cast includes hybrid mana symbols, the
player announces the nonhybrid equivalent cost he or she intends to
pay. If a cost that will be paid as the spell is being cast includes
Phyrexian mana symbols, the player announces whether he or she intends
to pay 2 life or the corresponding colored mana cost for each of those
symbols. Previously made choices (such as choosing to cast a spell
with flashback from a graveyard or choosing to cast a creature with
morph face down) may restrict the player’s options when making these
choices.
601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative
costs. Some effects may increase or reduce the cost to pay, or may
provide other alternative costs. Costs may include paying mana,
tapping permanents, sacrificing permanents, discarding cards, and so
on. The total cost is the mana cost or alternative cost (as determined
in rule 601.2b), plus all additional costs and cost increases, and
minus all cost reductions. If multiple cost reductions apply, the
player may apply them in any order. If the mana component of the total
cost is reduced to nothing by cost reduction effects, it is considered
to be {0}. It can’t be reduced to less than {0}. Once the total cost
is determined, any effects that directly affect the total cost are
applied. Then the resulting total cost becomes “locked in.” If effects
would change the total cost after this time, they have no effect.
